Semiconductor chemical processes are often performed on substrates, such as semiconductor wafers, at elevated temperatures. In certain processes it is desirable to perform various process steps with the substrate at different temperatures. One example of such a process is the removal of ion implanted photoresist, in which the ion implanted material forms a crust in the outer skin of the photoresist. The crust thickness and makeup depends upon the acceleration voltage of the ions, beam current, total dose and the thermal curing temperature of the original photoresist.
If the temperature of the implanted photoresist exceeds a certain temperature, solvents in the underlying photoresist vaporize and explode or “pop” through the crust. The popped crust material tends to spread throughout the inside of the process chamber, creating a substantial amount of particulate and leaving a residue on the chamber walls that is very difficult to remove. Additionally, it can be very difficult to remove the residues that form on the substrate at the popping sites. Often removal of these residues requires an expensive wet chemistry follow-on step.
The temperature at which the crust pops is usually substantially similar to the original photoresist curing temperature. Photoresist is spun onto a wafer in liquid form and then cured via a baking step on a hot plate. Typically, different curing temperatures are chosen for different applications, but curing temperatures generally vary from a low of about 80° C. to a high of about 180° C. or, in some cases, more than 200° C. A low curing temperature usually results in a similarly low implanted photoresist popping temperature.
In order to strip photoresist under the crust material efficiently, it is desirable to elevate the substrate temperature to above about 200° C. and preferably between about 250° C. and 300° C. The addition of a fluorine-bearing gas, such as CF4, can accelerate photoresist stripping at lower temperatures, but the fluorine tends to attack silicon dioxide features on the substrate as well. It is normally extremely undesirable to permit silicon dioxide loss. Accordingly, when using a fluorine gas compound, the amount used is carefully selected depending upon the substrate temperature. Larger percentages of CF4 are permitted when the substrate is at a low temperature, such as between about 25° C. and 80° C. However, when the temperature is elevated above 250° C., for example, the fluorine becomes extremely aggressive toward the silicon dioxide, and unacceptable amounts of the silicon dioxide are removed. In addition, fluorine gas compounds are often used for removing residues left on the substrate after the ion implant step. Therefore, to effectively use fluorine-bearing gas compounds, the substrate temperature must be carefully controlled relative to the percentage of fluorine gas compounds present in the chamber.
The conventional method of heating the substrate when the temperature is constantly cycled is to use lamps or other radiant energy sources. The substrate is generally heated from about room temperature to a predetermined temperature that is just below the popping temperature of the crust material. Even a relatively high percentage of CF4 or other fluorine-bearing gas compound, (typically about 3 percent to about 15 percent of total process gas flow), can be used to remove the ion implanted crust without causing serious damage if the temperature is kept relatively low.
Once the first process step of removing the ion implanted crust is complete, the substrate temperature can be raised to normal photoresist strip temperatures of between about 200° C. and 300° C., preferably about 250° C. to 300° C. Often, very low concentrations of fluorine-bearing gases, as little as 0.2 percent to 1 percent, are used at these higher temperatures.
One problem with using radiant lamp-based heating techniques is the difficulty in maintaining temperature uniformity across the substrate. There is often considerable variation in the radiant heating pattern on the substrate, which can lead to similar variations in the rate of processing across the substrate, so that some areas of the substrate will be further along in the process than other parts of the substrate. As a result, the chemical process time must be extended to be certain the areas which are heated slower have had time to complete the processing. This additional process time, however, also causes greater silicon dioxide loss. As critical geometry sizes shrink in advanced semiconductor technology, even a few Angstroms of silicon dioxide loss may be unacceptable.
An alternative substrate heating method is to use a thermal chuck to heat the substrate and photoresist layer. However, the temperature of the massive thermal chuck generally cannot be changed as quickly, thereby substantially increasing process times. Alternatively, the chuck can be held at a constant temperature that is below the ion implanted photoresist popping temperature. While the relatively large mass of the heated chuck provides a very uniform heat transfer to the substrate, the total process time is generally substantially longer than a lamp based system due to the reduced wafer temperatures. Thus, it is generally economically preferred to use a lamp-based system for heating the wafers due to the increased throughput, despite the undesired non-uniformity issues mentioned above.
Accordingly, it is desirable to combine the benefits of the heating uniformity achieved by a thermal chuck with the wafer temperature variability conventionally achieved by lamp-based wafer heating systems. Thus, a need exists for improved systems and methods for rapidly and uniformly changing the temperature of a substrate during processing.